1. Field of the invention
The present invention is related to a light emitting element and a flat panel display having a diamond film that can achieve a high brightness with low electricity consumption.
2. Description of the Related Art
Diamond is known to have excellent resistance to high temperature, have a large band gap (5.5 eV), and hence is electrically a good insulator when undoped. However, diamond can be semiconducting by doping suitable impurity atoms in the diamond. Furthermore, diamond has excellent electrical properties such that the breakdown voltage is high, the saturation velocities of carriers (electrons and holes) are also high, and the dielectric constant, and hence the dielectric loss, is small. It is also well known that diamond has the highest thermal conductivity among all materials at room temperature, and the specific heat is small.
Regarding chemical vapor deposition (CVD) of diamond film, the following techniques are known: microwave plasma CVD (for example, see Japanese patents (Laid Open) Nos. Sho 59-27754 and Sho 61-3320), radio-frequency plasma CVD, hot filament CVD, direct-current plasma CVD, plasma-jet CVD, combustion CVD, and thermal CVD. By these techniques, it is possible to form continuous diamond films over a large area at low cost on substrates of non diamond materials.
Recently, a vacuum field emission-type light emitting clement was proposed that consists of an electrode coated with a fluorescent material that faces a diamond film in vacuum. In the light emitting element, electrons are emitted from the diamond film, travel through vacuum, are accelerated toward the electrode under a high voltage between the diamond film and the electrode, and the light emission takes place in the fluorescent material due to the electronic excitation by the injected high energy electrons. Also, light emitting elements using silicon or metals, instead of diamond film, have been proposed (see J. Ito, "Vacuum micro-electronics", Oyo Butsuri, Vol. 59, No. 2 (1990), and K. Yokoo, "Vacuum microelectronics, the world of new vacuum devices", Journal of IEEE Japan, Vol. 112, No. 4 (1992)).
FIG. 12 shows an example of a cross-sectional view of a light emitting element using silicon, referred to as "Background Art 1". In FIG. 12, a conducting silicon layer 1 is formed on an insulating substrate 12, and then a cone-shape electron emitter 2 is formed on the surface of the silicon layer by microfabrication. A fluorescent electrode 6 is placed to oppose the emitter 2 across from the vacuum 7. The fluorescent electrode 6 is formed by successively depositing a transparent electrode 4 and a fluorescent thin film 5 on a transparent plate 3. The transparent electrode 4 and the silicon substrate 1 are connected to a power supply 9 to apply a voltage between them.
In the light emitting element according to Background Art 1 (FIG. 12), electrons 8 are emitted from the silicon electron emitter 2 toward the fluorescent electrode 6 by applying an electrical voltage between the fluorescent electrode 6 and the silicon substrate 1. The electrons 8 then electronically excite the fluorescent thin film 5 to make it fluoresce.
FIG. 13 shows a cross-sectional view of a light emitting element with a gate electrode for a flat panel display using silicon. This will be hereafter referred to as "Background Art 2". The difference between the light emitting element shown in FIG. 13 and that shown in Background Art 1 lies in the use of an insulating layer 11 formed around the emitter 2 on the silicon substrate 1, and a gate electrode 10 surrounding the emitter 2 formed on the insulating film 11. The flow of electrons 8, and hence the brightness of the fluorescence light from the fluorescent thin film 5, can be controlled by changing the voltage at the gate electrode 10.
In Background Arts 1 and 2, the fluorescence colors can be arbitrarily controlled by selecting a suitable material for the fluorescent thin film 5. It is also possible to fabricate a flat panel display from a two-dimensional array of the light emitting elements.
However, as presently appreciated, there is a problem in Background Arts 1 and 2 in that electron emission characteristics deteriorate shortly after the operation. This is attributed to the silicon, used for the electron emitter 2, not being sufficiently resistant to heat. As a result, the tip of the electron emitter 2 is easily rounded by the heat generated during the operation, which consequently reduces the gradient of the electric field near the tip, and hence the electron emission. The electron emission characteristics also deteriorate because of silicon oxidation by residual oxygen in the vacuum gap 7 of FIGS. 12 and 13. Oxygen is known to easily react with silicon to form an insulating SiO.sub.2 layer on the surface of the electron emitter 2 and increase its work function. For those reasons, a silicon emitter has never been employed for practical use because the emitter lifetime is not sufficiently long, and the silicon emitter can not sustain high electric power.
There is another problem of non-uniform brightness across the display in the vacuum field emission-type display because it is very difficult to maintain a constant vacuum gap 7 between electron emitters and electrodes within a micron-order precision over the entire area of the display.
The problems stated above are more or less similar for metal emitters, and can not be completely solved by using any materials for the electron emitter. The essential cause of the above problems lies in the fact that the vacuum gap 7 exists between the emitter 2 and the fluorescent electrode 6 in Background Arts 1 and 2.
It is well known that diamond exhibits a good electron emission under a negative voltage (see, C. Wang et al, Electronics Letters, Vol. 27, No. 16, p. 1459, (1991)), and thus diamond particles and films grown by CVD are currently investigated as a promising material for high performance electron emitter applications. However, the electric current from diamond is only on the order of 10 mA/cm.sup.2, significantly smaller than the typical value, 1000 mA/cm.sup.2, for an integrated silicon electron emitter array.
It was also reported that electrons in diamond can drift without energy loss due to electron-phonon interaction in a high electric field greater than 10.sup.4 V/cm (see, Z.-H. Huang et al, Applied Physics Letters, Vol. 67, No. 9. p. 1235 (1995)).
The present invention is proposed to solve above stated and problems. It is an object of the present invention to provide a light emitting element and a flat panel display having a diamond film that achieve a stable and high light emission with low electricity consumption because no vacuum gap is required between the lower electrode and the fluorescent film in the present device structure.